1. Field of the Invention
The present invention relates to a dust collecting apparatus for dedusting a Ca-containing gas by a ceramics filter and an operation method thereof.
2. Description of the Prior Art
A ceramics filter is appropriate for collecting dust contained in a high temperature gas. Especially in a pressurized fluidized-bed combustion power generation system as shown in FIG. 7, the ceramics filter provided therein is effective for reducing the wear rate of the gas turbine blade material and reducing dust in the combustion waste gas to be discharged into the air.
A prior art dust collecting apparatus will be described with reference to FIG. 7. In FIG. 7, coal 101, supplied air 102 and a desulfurizing agent 103 are supplied into a pressurized fluidized-bed combustion furnace 1. The desulfurizing agent 103 is first supplied into a hopper 15 through a valve 14 which is open. Then the valve 14 is closed and the hopper 15 is pressurized by a gas (not shown) so that a pressure in the hopper 15 and that in a hopper 13 become equal to each other. Then a valve 12 is opened and the desulfurizing agent 103 is caused to fall into the hopper 13. The desulfurizing agent 103 is metered by a feeder 16 and is carried with a gas flow to be mixed into air 105 so that a mixture 104 of the air and the desulfurizing agent is supplied into the pressurized fluidized-bed combustion furnace 1.
The coal 101 is fluidized by the supplied air 102 to be combusted. SO.sub.2, which is generated by the combustion, reacts with and is absorbed by the desulfurizing agent 103.
A combustion gas 201 generated at the pressurized fluidized-bed combustion furnace 1 is dedusted by a cyclone 2. Dust 202 which is removed from combustion gas 201 is discharged out of the system. Combustion gas 301, after being dedusted, bifurcates to enter a filter container 3a, 3b respectively. In the filter container 3a, 3b,there are provided a multiplicity of ceramics filters 31a, 31b. Each of the ceramics filters 31a, 31b is tubular and is constructed in such a gas flow structure that a cyclone outlet gas (combustion gas) 302, 303 is led thereinto and the combustion gas passes therethrough from an inside to an outside thereof.
When the combustion gas 302, 303 passes through the ceramics filter 31a, 31b, the dust contained in the combustion gas 302, 303 is collected on an inner surface of the ceramics filter 31a, 31b. The dust so collected on the ceramics filter is peeled off by a back wash gas 306, 307, which flows periodically, and falls down in the ceramics filter 31a, 31b to a bottom portion of the filter container 3a, 3b to be recovered therefrom at 308, 309. In a buffer tank 33, there is stored a pressurized air 304, thus the back wash gas 306, 307 is supplied into the filter container 3a, 3b by opening an closing a valve 32a, 32b periodically so as to allow the pressurized air from the buffer tank 305 to flow through valve 32a, 32b.
Combustion gases 401, 402, having passed through the ceramics filter 31a, 31b, joins together outside of the filter containers 3a, 3b, to form a combustion gas 403, which is introduced into a gas turbine 4. The combustion gas 403 drives the gas turbine 4 to thereby generate an electric power by a generator 10. A combustion gas 501 at the gas turbine outlet is supplied into a waste heat recovery boiler 5 so that a sensible heat of the combustion gas 501 is converted into steam energy 701 by a heat exchanger 9. The steam 701 drives a steam turbine 7 to thereby generate electric power by a generator 11. Steam 801 which has come out of the steam turbine is changed to become a condensate by a condenser 8 and water 901 thereof is pressurized again to be supplied to the heat exchanger 9 of the waste heat recovery boiler 5. The combustion gas 601 which has passed through the waste heat recover boiler 5 is discharged into the air from a stack 6.
In the pressurized fluidized-bed combustion power generation system, if a load is increased, the temperature of the combustion gas 301 is elevated corresponding to the load. Thus, the temperature of the ceramics filter 31a, 31b is also elevated. Generally that temperature is approximately 650.degree. C. at a load of 50%, approximately 750.degree. C. at a load of 75% and approximately 830.degree. C. at the load of 100%. When a B type limestone is used for the desulfurizing agent 103 in the prior art system shown in FIG. 7, no change over time is caused in the differential pressure in the ceramics filter 31a, 31b at the temperature of 650.degree. C. However, in a case where the load is increased and the temperature of the ceramics filter 31a, 31b exceeds 750.degree. C., a phenomenon is caused in which the differential pressure in the ceramics filter 31a, 31b is elevated over time. If the temperature is set to a temperature at which the differential pressure in the ceramics filter 31a, 31b starts to become elevated, the differential pressure in the ceramics filter 31a, 31b continues to increase until the operation must be finally stopped.
Elevation of the filter back wash pressure is effective as one of the methods for reducing the filter differential pressure. However, to elevate the filter back wash pressure invites a breakage of a seal portion of the ceramics filter 31a, 31b or a breakage of a pressure structure portion of the filter container 3a, 3b. Hence, there is an upper limit value in the filter back wash pressure. Even if the back wash pressure is set to a maximum back wash pressure within a permissible range, if the B type limestone is used and the filter temperature is between 750.degree. C. and 810.degree. C., the filter differential pressure continues to increase over time, and there is a problem that the operation of the pressurized fluidized-bed combustion power generation system must be finally stopped.